The present inventions relate generally to tools used in subterranean wells and, in a preferred embodiment thereof, more particularly provide a slurry delivery apparatus for use in formation fracturing operations.
Further, the present inventions relate to improvements in prior art slurry delivery tools and methods of the type shown and described in U.S. Pat. No. 5,636,691, entitled Abrasive Slurry Delivery Apparatus and Methods Of Using Same, which is incorporated by reference herein for all purposes.
As is explained in the above referenced patent, oftentimes, a potentially productive geological formation beneath the earth""s surface contains a sufficient volume of valuable fluids, such as hydrocarbons, but also has a very low permeability. xe2x80x9cPermeabilityxe2x80x9d is a term used to describe that quality of a geological formation that enables fluids to move about in the formation. All potentially productive formations have pores, a quality described using the term xe2x80x9cporosity,xe2x80x9d within which the valuable fluids are contained. If the pores are not interconnected, the fluids cannot move about and, thus, cannot be brought to the earth""s surface.
When such a formation having very low permeability, but a sufficient quantity of valuable fluids in its pores, is desired to be produced, it becomes necessary to artificially increase the formation""s permeability. This is typically accomplished by xe2x80x9cfracturingxe2x80x9d the formation, a practice which is well known in the art, and for which purpose many methods have been conceived. Fracturing is achieved by applying sufficient pressure to the formation to cause the formation to crack or fracture, hence the name. The desired result being that the cracks interconnect the formation""s pores and allow the valuable fluids to be brought out of the formation and to the surface.
A conventional method of fracturing a formation begins with drilling a subterranean well into the formation and cementing a protective tubular casing within the well. The casing is then perforated to provide fluid communication between the formation and the interior of the casing. A packer is set in the casing above the well treating equipment to isolate the formation from the rest of the wellbore. In some environments, it is preferable to use a packer of the type that is set using a ball-seat configuration. Dropping a ball through the well tubing to a seat in the tubing string located at the packer sets these packers. The ball acts as a temporary check valve-closing seat, permitting pressure within the tubing string to be increased at the packer to hydraulically set (install) the packer. After the packer is set, tubing pressure is increased further to a point where the ball seat fails and allows the ball to fall down the tubing string to reopen the tubing at the packer. Some fracturing equipment contains internal components made from corrosion resistant material such as carbide or ceramic which cannot tolerate impact with a ball being projected downhole when the seat fails. Some have no space for receiving and storing the ball. In those situations, the ball must be removed by reversing the flow in the well, to flow the ball from the well, rather than by failure of the seat. The process of reverse flow to remove the packer set ball is a time consuming and expensive process, which should be avoided.
In some applications, the tubing string installed with the packer includes perforating equipment located below the fracturing equipment. One type of perforating equipment uses explosive charges that are conventionally actuated by dropping a weighted bar through the tubing string. If the fracturing equipment contains fragile internal components, the weighted bar actuating system cannot be used.
After the packer is set, hydraulic pressure is applied to the formation via tubing extending from the packer to pumps on the surface. The pumps apply the hydraulic pressure by pumping fracturing fluid down the tubing, through the packer, into the wellbore below the packer, through the perforations, and finally, into the formation. The pressure is increased until the desired quality and quantity of cracks is achieved and maintained. Much research has gone into discerning the precise volume and rate of fracturing fluid and hydraulic pressure to apply to the formation to achieve the desired quality and quantity of cracks.
The fracturing fluid""s composition is far from a simple matter itself. Modem fracturing fluids may include sophisticated manmade proppants suspended in gels. xe2x80x9cProppantxe2x80x9d is the term used to describe material in the fracturing fluid which enters the formation cracks once formed and while the hydraulic pressure is still being applied (that is, while the cracks are still being held open by the hydraulic pressure), and acts to prop the cracks open. When the hydraulic pressure is removed, the proppant keeps the cracks from closing completely. Thus, the proppant helps to maintain the artificial permeability of the formation after the fracturing job is over. Fracturing fluid containing suspended proppant is also called xe2x80x9cslurry.xe2x80x9d
A proppant may be nothing more than very fine sand, or it may be a material specifically engineered for the job of holding formation cracks open. Whatever its composition, the proppant must be very hard and strong to withstand the forces trying to close the formation cracks. These qualities also make the proppant a very good abrasive. It is common for proppant to form holes in the protective casing, tubing, pumps, and any other equipment through which slurry is pumped.
Particularly susceptible to abrasion wear from pumped slurry is any piece of equipment in which the slurry must make a sudden or significant change in direction. The slurry, being governed by the laws of physics and fluid dynamics including the principles of inertia, tends to maintain its velocity and direction of flow, and resists any change thereof. An object in the flow path of the slurry, which tends to change the velocity or direction of the slurry""s flow will soon be worn away as the proppant in the slurry incessantly impinges upon the object.
Of particular concern in this regard is the piece of equipment attached to the tubing extending below the packer, which takes the slurry as it is pumped down the tubing and redirects it radially outward so that it exits the tubing and enters the formation through the perforations. That piece of equipment is called a crossover. Assuming, for purposes of convenience, that the tubing extends vertically through the wellbore, and that the formation is generally horizontal, the crossover must change the direction of the slurry by ninety degrees. Because of this significant change of direction, few pieces of equipment (with the notable exception of the pumps) must withstand as much abrasive wear as the crossover.
In addition, the crossover is frequently called upon to do several other tasks while the slurry is being pumped through it. For example, the crossover typically contains longitudinal circulation ports through which fracturing fluids, that are not received into the formation after exiting the crossover, are transmitted back to the surface. Space limitations in the wellbore dictate that the circulation ports are not far removed from the flow path of the slurry through the crossover. If the crossover is worn away such that the slurry flow path achieves fluid communication with the circulation ports in the crossover, the fracturing job must cease. Once stopped, the fracturing job cannot be recommended or completed. Hence, it is very important that the crossover does not fail while the job is in process. If the fracturing job is not halted after the crossover fails, the slurry will enter the circulation ports in the crossover and travel back to the surface without delivering the proppant to the formation.
For the above reasons and others, the crossover has commonly been considered a disposable piece of equipment, usable for only one fracturing job, or worse, less than one fracturing job. Even when it survives a fracturing job, it is usually sufficiently worn that no further use may be made of it. This is unfortunate because the crossover is also typically one of the most expensive pieces of equipment used in a fracturing job due to its high machining and material costs. Further, customers are now demanding fracturing jobs with high flow rates, high pressures, higher volumes, and higher density proppant. All of this increase wear on the crossover increases the likelihood of crossover failure during the fracturing job.
Attempts have been made to provide a solution for these problems. One involves making the crossover out of extremely hard and abrasion wear-resistant materials. This has proven to reduce the rate of abrasion wear of the crossover.
It is, however, enormously expensive to make an entire crossover out of a sufficiently wear-resistant material. No economic advantage is actually achieved by this solution over the disposable crossover made of less wear-resistant, but much less expensive, materials.
Another proposed solution is to utilize surface treatment of less expensive alloy steels to achieve a wear-resistant crossover surface. Methods such as carburizing, nitriding, etc., which produce a high surface hardness, do indeed slow the abrasion wear rate of the crossover at less expense than using exotic materials. However, as soon as the hardened surface layer has been breached, the crossover begins to wear away rapidly. For this reason, surfacehardened crossovers are also not sufficiently durable for the newer high flow, high pressure fracturing jobs. The extra expense of surface-hardening a disposable crossover makes this solution uneconomical as well.
Other proposals involve placing replaceable protective sleeves or liners of wear or corrosive resistant materials in the crossover. These sleeves are placed in the areas subjected to abrasion such as the central tubing passageway. However, as previously pointed out, the better corrosive resistant materials are brittle and subject to impact damages from contact with packer setting balls and perforator activator bars. This potential for damage limits the use of these crossovers. Further protective sleeves take up space in the internal flow path, reducing the flow capacity and tool passage ability of the crossover.
Another area of concern concerning abrasion wear during fracturing jobs is the protective casing lining the wellbore. Since the crossover typically directs the slurry flow radially outward, the casing is directly in the altered slurry flow path. Unintended, misplaced holes in the casing are to be avoided, since it is the casing which provides the only conduit extending to the surface through which all other conduits and equipment must pass.
From the foregoing, it can be seen that it would be quite desirable to provide an improved slurry delivery apparatus, which does not have the economic disadvantages of the solutions, enumerated above, but which allows repeated use thereof. It would also be desirable to provide a slurry delivery apparatus that has a full bore internal passage to increase flow capacity and tool clearance. In addition, it would be desirable to locate replaceable wear resistant components out of the main passageway and out of potential tool damaging areas. Accordingly, it is an object of the present invention to provide such a slurry delivery apparatus and associated methods of using same.
In carrying out the principles of the present invention, in accordance with an embodiment thereof, an abrasive slurry delivery apparatus and method of using same are provided, which apparatus and method are specially adapted for utilization in formation fracturing operations in subterranean wellbores.
In broad terms, an abrasive slurry delivery apparatus and method is provided which includes a first tubular structure having a flow inlet communicating with an internal flow passage through which a pressurized, abrasive slurry material may be axially flowed in a downstream direction. The internal flow passage is positioned eccentrically within the tubular structure, to form a thickened axially extending wall section. An axially extending bypass flow passage formed in the thickened wall section, and an axial portion of the tubular structure having a side wall section, with an outlet opening therein through which an abrasive slurry material may be outwardly discharged from the internal flow passage. The outlet opening being circumscribed by a peripheral edge portion of the side wall section, and protective means for shielding the peripheral edge portion of the side wall section from abrasive slurry material being discharged outwardly through the outlet opening, with or without the protective means of abrasive resistant ported inserts removably or permanently mounted in the side wall flush with or recessed from the internal flow passage. It is preferable but not required that these outlet openings will be aligned such to promote maximum volume with equal distribution of flow throughout the length of the crossover with specific spacing, number and size of the ports. These outlet holes shall encompass the entire surface area (radially) with the possible exception of that area left for the bypass return ports.